Weathering of Quartz As an Archean Climatic Indicator

Earth and Planetary Science Letters 241 (2006) 594–602 www.elsevier.com/locate/epsl Weathering of quartz as an Archean climatic indicator Norman H. Sleep a,*, Angela M. Hessler b a Department of Geophysics, Stanford University, Stanford, CA 94305, USA b Department of Geology, Grand Valley State University, Allendale, MI 49401, USA Received 25 July 2005; received in revised form 4 November 2005; accepted 11 November 2005 Available online 13 December 2005 Editor: K. Farley Abstract Chert and other hard monomineralic quartz grains weather mostly by mechanical processes in modern environments. Their clasts are overrepresented in conglomerates and sands relative to their sources regions. Conversely, macroscopic dissolution features, including quartzite karst, are rare but not nonexistent. The similar rarity of quartz dissolution in Archean deposits provides a paleothermometer for climate on the early Earth. For example, chert is overrepresented in conglomerates and sands of the ~3.2 Ga Moodies Group (South Africa) relative to the source region. Features related to the far-from-equilibrium dissolution rate are particularly diagnostic as it increases an order of magnitude over 25 8C, much more than solubility. Extrapolating from observed dissolution rates in modern environments that weather at ~25 8C, we expect obvious dissolution features in ancient climates above ~50 8C. Polycrystalline quartz and chert would readily disaggregate by solution along grain boundaries, yielding silt and clay. Quartz grains within slowly weathering granite would become friable, yielding silt and clay, rather than sand. At still higher temperatures, Al2O3-rich clays from weathered granite would stand above solution- weathered chert on low-relief surfaces. The observed lack of these features is evidence that the Archean climate was not especially hot. D 2005 Elsevier B.V. All rights reserved. Keywords: chert; quartz sand; conglomerate; dissolution; karst; kinetics; climate; precambrian 1. Introduction See the work of Young et al. [4] for a possible exception at 2.9 Ga. Climate during the Archean Era (~3.8 to 2.5 Ga) is Theoretical studies of Archean temperature model a relevant to the history of life on Earth. However, only greenhouse atmosphere composed of carbon dioxide limited geological analysis bears on the topic. Oxygen- and methane. The commonly cited upper limit, 85 8C, isotope studies of cherts provide oceanic temperature is model dependent [5–8]. Conversely, reasonable estimates for 3.5–3.2 Ga of 55–85 8C [1]. Evidence of amounts of these gases do not necessarily imply tem- thorough weathering in the Archean, despite the absence peratures much higher than those of the modern tropics of vascular plants, has prompted temperature estimates or the Cretaceous Earth [8]. Limited direct estimates of as high as 85 8C [2,3]. In addition, the limited rock record atmospheric CO2 come from observations that siderite shows that glaciation was uncommon in the Archean. is present in 3.2-Ga pebble weathering rinds [9] but is absent from a 2.75-Ga paleosol [10] (Fig. 1). * Corresponding author. In this paper, we suggest that the weathering beha- E-mail address: [email protected] (N.H. Sleep). vior of quartz in monomineralic quartzite and chert and 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.11.020 N.H. Sleep, A.M. Hessler / Earth and Planetary Science Letters 241 (2006) 594–602 595 the mechanically derived conglomerates or poorly frac- tionated volcaniclastic deposits underlying the Moodies Group. 2. Archean geological observations Hessler studied well-preserved 3.2-Ga clastic sediments from Barberton Mountain Land for her thesis [12]. We concentrate on Moodies Group rocks north of the Inyoka Fault, which have been the subject of extensive geological work [13–17]. A striking feature of these rocks is that the degree of weathering prior to fluvial deposition is within the range of that observed at the warm, humid end of the climate spectrum on the modern Earth. This observation is elaborated below for each sedimentary size fraction within the Moodies Group. We cite other analogous localities from the Mesoarchean and Neoarchean. Fig. 1. Stability diagram of siderite in terms of weathering temperature and the partial pressure of CO2 from Rye et al. [10]. The typical 2.1. Conglomerate weathering temperature is ~10 K hotter than the global mean annual temperature [24]. Our 3.2-Ga pebbles [9] weathered under conditions on the upper left of the diagram while the paleosol of Rye et al. [10] The conglomerate data are straightforward (Fig. 2). weathered at 2.75 Ga under conditions on the lower left. Both situa- At the bottom of the section, interpreted to be alluvial tions are consistent with clement Archean climate. The present (pre- fan and braided river deposits [13,16], conglomerate À 6 industrial) CO2 pressure is ~300Â10 bar. clasts are poorly sorted, spherical, mostly well rounded, and have an average diameter of ~3 cm [9,12]. Chert as grains in granitic rocks is a viable Archean clasts of various colors dominate the pebble fraction of paleothermometer. The tendency on the modern Earth the rock. The rounded clasts upon visual inspection do for quartz to weather mechanically, unlike the dissolu- not have embayments as expected from dissolution. tion weathering of carbonates, is notorious. bAs unre- Although the conglomerates are locally variable, chert lenting flint to drops of rain.Q (Shakespeare, Titus clasts dominate across the study area (42–58%), despite Andronicus, Act II, Scene III). bHe plies her hard; chert covering b2% of the source region. Other rocks and much rain wears the marble.Q (Shakespeare, The types that are expected to resist chemical weathering are Third Part of King Henry the Sixth, Act III, Scene II). also overrepresented compared to the source region. There is an urge to regard evidence of this difference on These include quartz-rich sandstone and sericite schist. the ancient Earth as unremarkable. Rather, quartz does Quartz-poor sandstone, ultramafic and mafic volcanic dissolve some in modern environments [11]. Its solu- rocks, and shale are underrepresented. All the under- bility and dissolution rate of quartz are strong functions of temperature. Quartz will dissolve and chemically disaggregate readily in a sufficiently hot climate, as has been suggested for the Archean. Instead, field observations indicate that quartz in the Archean mostly resisted chemical weathering, like it does under modern clement conditions. Here, we quantify the effect using solubility and kinetic data to extrapolate from the chemical behavior of quartz observed in modern climates. Because the Barberton sequence (like most Mesoarchean sequences) is predominately syntectonic, we restrict our attention to sediments derived during Fig. 2. Drill core from the Sheba Gold mine in the Barberton area shows the abundance of chert pebbles (marked C) in a 3.2-Ga periods of relatively slow exhumation, such that chem- Moodies Group conglomerate. The red arrow points to a well-rounded ical weathering could proceed. We eschew stratigraphic mechanically weathered clast. Fractures postdate deposition. U. S. levels dominated by highly immature sediments, like quarter (25 mm) gives scale. 596 N.H. Sleep, A.M. Hessler / Earth and Planetary Science Letters 241 (2006) 594–602 represented rock types, but shale (which is mechanical- nearby Kaap Valley pluton (Fig. 3). Moodies sands ly weak), are expected to weather chemically. are less weathered than those generated in modern, The predominance of chert where it occurs in the extremely humid environments: qualitatively between source region and the mechanical weathering of its the fluvial arkoses of the warm–temperate to subhumid cobbles seem to be a general characteristic of mid- Mallacoota Basin [20] and the quartz–arenite sands Archean sequences (Roger Buick personal communica- produced in the seasonally tropical Orinoco Basin, tion, 2003). An example is 3.5-Ga deposits in Australia Venezuela [21]. [18]. Most of the shale fraction appears to have bypassed the exposed region of deposition. Enough shale exists, 2.2. Sandstone and shale however, to demonstrate that extensive chemical weathering did occur. Al2O3 ranges between 18 and Abundant quartz sandstone in the basal 700–800 m 25 wt.%, above that in any reasonable source rock. of the Moodies Group indicates a braided alluvial plain Conversely, detrital chert comprises less than 2% of [13,16]. Framework grains include abundant monocrys- the shale material. talline quartz (38–39%), potassium feldspar (9–15%), Although analogous data from sequences contempo- chert (9–15%), and quartz–sericite lithic grains (6– raneous with the Moodies Group is lacking, extensive 18%), depending on the location within the Barberton chemical weathering is also a common feature of Greenstone Belt. Less common framework grains in- Mesoarchean and Neoarchean sediments in the Cana- clude plagioclase, polycrystalline quartz, felsic and dian Shield [2,3,22–24] and South Africa [25]. Sand- mafic–ultramafic volcanics, lithic grains, and sedimen- stones in the ~2.6 Ga Beaulieu Formation [3] and the tary lithic grains. The modal composition plots in the ~2.6 Keskarrah Formation [2] show intense chemical recycled orogen field of Dickinson and Suczek [19] weathering in a humid climate, which the authors in- (Fig. 3). It is important to note that chert is overrepre- terpret as being as hot as 85 8C. These samples, how- sented in our sands relative to the source region, but to ever, are more quartz-rich than those in the Moodies a lesser degree than in the conglomerates. Group, plotting in the recycled orogen field of Dick- Overall the degree of weathering is greater than that inson and Suczek [19] at 62–90% [3] and 68–95% [2] observed in sands from modern rivers draining the quartz. As is expected, plagioclase and potassium feldspar are underrepresented relative to the assumed source region. Notably much of the quartz is polycrystalline, 85% and 56% of the grains, respectively, in the composite average.

Load more